1. Field of the Invention:
The present invention relates generally to improved methods and apparatus for measuring distance to a target, velocity of the target, and composition or identity of the target, which can be a solid, liquid or vapor. Transceiver energies used with the method may be in the acoustic energy band or in the optical, x-ray, radio-frequency, microwave, millimeter-wave or other electromagnetic energy bands.
2. Description of the Prior Art:
An urgent need exists in many industries, disciplines and governmental interests for a method capable of rapidly and precisely measuring any combination of distance, velocity, or composition or identity of a remote target. To be practical, such a method must be usable with most or all types of acoustic or electromagnetic transmitters, such as ultrasonic, laser or microwave transmitters; it must utilize relatively simple instrumentation and components for fabrication; it must provide high resolution at both short and long range; it must provide high discrimination against background noise; it must require relatively low transmitter powers; and it must be capable of correcting for non-optimal system performance and measurement conditions.
For the typical example of laser ranging, the prior art is substantially represented by only two fundamental ranging principles. One is time-of-flight (TOF) ranging and the other is continuous wave (CW) ranging. The fundamental concepts represented by these methods and by apparatus based on these methods are essentially similar for numerous applications and devices used in other bands of the electromagnetic spectrum, such as the microwave band, and in acoustic bands, such as the ultrasound band. Accordingly, for the sake of clarity, we will focus description of prior art primarily on laser ranging.
Although the approaches represented by TOF and CW ranging are well represented commercially and in the technical literature, their respective performance characteristics are less than desired. In both approaches, instantaneous ranging that is both highly precise and unambiguous at short and long range is essentially impossible due to fundamental limitations. Specifically, these prior methods operate as follows:
TOF Laser Ranging.
A short pulse of energy emitted from a transmitter is used to illuminate a target, and a portion of the transmitter signal returning from the target is subsequently detected using a high speed receiver. The temporal delay between transmission and detection is measured to determine range, based on the relationship, R=ct/2, where R is the distance from the transceiver to the target, c is the speed of light, and t is elapsed time between transmission and reception. Range resolution, .DELTA.R, is related to rise time of the laser pulse, .tau..sub.r, and system signal-to-noise ratio, SNR, according to the relationship, .DELTA.R=c.tau..sub.r /(2 SNR), where .tau..sub.r is equal to approximately 0.7 times the pulse width, and a typical minimum SNR at maximum range is 8. Typical values for .DELTA.R are 50-500 mm at maximum range, R.sub.MAX. A low transmitter duty cycle must be used to prevent overlap of return signals from distant targets. Accordingly, the maximum unambiguous range, R.sub.AMB, is related to pulse repetition frequency, PRF, by the relationship, R.sub.AMB =c/2 PRF (at PRF=10 kHz, R.sub.AMB =15 km). High speed, incoherent detection systems are commonly used (where the detector rise time &lt;&lt;.tau..sub.r), making discrimination against optical interferences and electronic noise relatively poor. Averaging of multiple transmitter pulses or use of high pulse energies can ameliorate the effects of such noise, but this approach generally dictates significant compromises in total measurement time and in eye-safety. In general, the TOF approach is most amenable to use at very long distances, where the compromises between large R.sub.AMB and large absolute .DELTA.R performance may not be a significant concern. Conversely, the TOF approach is poorly suited for measurement of targets at close range, where the magnitude of .DELTA.R becomes large relative to R.
In this simple example of TOF laser ranging, the optical frequency of the transmitted radiation constitutes a carrier frequency upon which an encoding pattern is imposed, specifically a binary off-on-off pattern that is emitted as a short burst of optical energy. The detector can be made primarily responsive to this carrier through the use of an optical bandpass filter in the receiving optics. In other energy bands, such as microwaves, the carrier might be a specific microwave frequency that is transmitted in the form of a burst of microwave energy at the specific frequency, while detector response might be limited primarily to that transmitted microwave carrier frequency using electronic bandpass filtering. In both cases, this mode of detection is referred to as incoherent (or direct) detection, because there exists no defined phase relationship between the transmitted signal and the received signal. Hence, phase-sensitive means cannot be used to discriminate noise at the carrier frequency, such as background light, that arrives at the detector during the measurement period.
CW Laser Ranging.
Range information is obtained by modulating a CW carrier frequency with a characteristic pattern, such as a sinusoidal amplitude pattern or a saw-tooth frequency or phase modulation envelope imposed on an optical carrier. Phase delay between the transmitted signal and the detected return signal is measured to determine range according to the relationship, R=.delta.c/2.function., where .delta. is the fractional phase shift between the transmitted signal and the return signal, c is the speed of light, and .function. is the modulation frequency of the laser. The wavelength of this modulation, .lambda., is related to .function. by the relationship, .lambda.=c/.function.. Range resolution is related to minimum measurable fractional phase shift, .DELTA..delta., and modulation frequency, .function., according to the relationship, .DELTA.R=.DELTA..delta.c/2.function.. Hence, if .function.=100 MHz and .DELTA..delta.=0.001, then .DELTA.R=1.5 mm. R.sub.AMB =.lambda./2=c/2.function. (when .delta..gtoreq.1). This is the case since phase delay is linearly proportional to distance: at long distance or at high modulation frequencies, phase shifts in excess of 2.pi. will occur that cannot be distinguished from degenerate solutions to the equation R=(.delta.+n)c/2.function., where n is an integer multiple of R.sub.AMB. Hence, for .function.=100 MHz, R.sub.AMB =1.5 m. To avoid range ambiguity arising from aliasing of the modulated signal, modulation frequency can be reduced (reducing ranging precision), or several modulation frequencies can be used in succession (increasing measurement time). For example, chirped or multiple modulation frequencies can be used to circumvent some of these limitations. A major advantage of CW ranging is that coherence can be maintained between the transmitter and detector based on the encoded modulation pattern. This allows strong rejection of incoherent noise from ambient sources, such as sunlight.
Coherent Laser Ranging.
Various optical and electronic coherence methods may be used to enhance the performance in both TOF and CW ranging. The term "coherence" refers to a measurement system that maintains a controlled phase relationship between the transmitted and detected signals, either at the fundamental carrier frequency of the transmitted electromagnetic radiation (for example, where optical coherence is achieved using 785 nm light, which corresponds to a carrier frequency of 3.times.10.sup.14 Hz) or by imposition of a coherent modulation frequency upon the carrier frequency (such as amplitude modulation of 785 nm light at 100 MHz). Either approach facilitates sensitive and selective detection of the transmitted signal by allowing phase-sensitive processing methods to be applied to the detected return. For example, optical mixing techniques are sometimes used to extract information from the return of a highly stable optical carrier, and may be useful for determining radial target velocity. In contrast, radio-frequency (RF) modulation of transmitter amplitude, frequency, or phase can be used to effect electronic coherence between a transmitted waveform and a reference oscillator. Mixing of the receiver signal with the reference allows phase sensitive demodulation to be performed. The two approaches may be combined, for example to obtain simultaneous range and Doppler information, while several advanced variations of these general methods have been used to further extend performance. These include: pseudo random modulation (RM) and phase modulation (PM), which represent further variations from conventional methods wherein temporal delay to maximum correlation is used to calculate range. These basic concepts have seen further development in the field of reflectometry, where various combinations of electronic and optical mixing have been used to optimize R.sub.AMB or .DELTA.R. Similar implementations in laser ranging are beginning to evolve. Other hybrid approaches, such as amplitude modulation of a frequency-chirped sub-carrier, are also being developed. Unfortunately, these approaches fail to simultaneously optimize R.sub.AMB and .DELTA.R, while the additional hardware overhead increases system cost and complexity.
Importantly, specific implementations of the aforementioned ranging methods may exhibit complications that cause the transmitted or received ranging signal to differ from optimal conditions, thereby resulting in reduced sensitivity or erroneous determination of range or other measurable parameters. Such complications may arise due to imperfections in instrumental design or operation, effects of target properties, or interference from atmospheric effects or spurious electromagnetic energies. Suppression methods for some such complications have previously been applied in other disciplines, such as the phase-alternation schemes described by Wachter et al. (International Journal of Mass Spectrometry and Ion Processes, 103 (1991) 169-179). However, such methods have not previously been applied to ranging methods and apparatus.
Numerous innovations for ranging have been provided in the prior art that are adapted to be used. Even though these innovations may be suitable for the specific individual purposes to which they address, they would not be suitable for the purposes of the present invention as heretofore described.
While the concepts of TOF and phase-based ranging have been taught using various combinations of an interrupted burst of energy at a carrier frequency, modulating this carrier, and using various coherent demodulation methods on the resultant signal, the advantages and implications of a new ranging method that combines the best features of each of these has not been previously taught nor appreciated. Moreover, the application of advanced methods for suppression of instrumental or environmental complications to such a new ranging method has not been previously taught nor appreciated.